I've been taught that when you lay shingles, the first (bottom) row on the eave should have a double row, and that the bottom one is placed upside down.
Why?
Why do you double the bottom row of asphalt shingles
roofshingles
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More important question: Which contractor did you feel the most comfortable with and which has the best work history in your area -- have you checked BBB and something like Angies List? Who has the best relationship with their wholesaler/supplier and can get you support if you need it in case of roof damage?
Owens Corning and GAF make comparable products; the Owens Corning warranty is longer, but neither warranty is good unless the installer installs it properly with the proper number of nails at the proper spacing. Specifically, the two products that you're looking at are almost identical. One difference besides the warranty is that Owens Corning has a constant bead of sealant on the backside of the shingles to seal them to the shingles underneath; if you're in an area that gets significant amounts of wind-driven rain (such as a coastal area) to the point that you will be testing the wind resistance of the shignle, you will PROBABLY want to choose the GAF shingle because the Owens Corning shingles sometimes have problems draining after water has blown up underneath them.
Remember that it isn't the shingle that matters; the shingle really just protects your roof. Your roof is actually the tar paper and other membrane (such as Grace Ice & Water Shield) that goes on under the shingles. Those products should be of the highest quality and have perfect installations. Getting good shingles is essential, but it's almost secondary if you cheap out on the underlayments or your roofer installs it incorrectly.
One note that I'm suggesting all homeowners explore: If your attic is hot during the summer, look into the 'Cool Roof' shingles that both OC and GAF produce. They have a reflective substance in with the stone and will help reflect solar energy back out before it has a chance to enter your roof structure.
Replace it now. Like TODAY. That's BAD.
With that kind of bad shingle wear, your shingles are likely to create ice dams which will cause water to back up and leak through the roof, causing interior damage, compounding the cost.
In the summer, the roof won't hold the water because it can't freeze, so the curls will only grab a very little water before it drips free.
In the winter, however, a little retained water will freeze, expand, and capture more water, which will freeze, and so forth.
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Best Answer
Because asphalt shingles are slottet and have gsaps at their ends. Those openings are covered by the solid top of the (offset) next row down. At the bottom, tho cover those spaces, you need the coverage provided by the next row down.
Rather than sdpent time cutting up shingles or getting a separate product for that space, we just use an additional nrow of shingles, offset but not moved fsarther down the roof.
Mounting them upside down is a matter of "as long as they're hidden anyway, let's use the unslitted half for that purpose and get slightly better coverage."
If you were using traditional separate shingles, you would still need to fill the gaps between shingles but presumably reversing them would have little effect on that. It might affect the angle or appearance of the bottom edge... But I honestly don't know offhand what standard practice is in that regard.